1. Field of the Invention
The present invention relates to a pipe fastening structure for use in a fuel line on an internal combustion engine, such as a diesel or a gasoline engine. More particularly, the present invention relates to a pipe fastening structure for fastening a spherical end of an end member attached to an end part of a metal pipe to a counterpart with a nut.
2. Description of the Related Art
A fuel line on an internal combustion engine, such as a diesel or a gasoline engine, includes a metal pipe having a spherical end for carrying high-pressure fuel. The spherical end of the metal pipe is fastened to a counterpart on the engine by a pipe fastening structure including a nut for fastening the spherical end to the counterpart on the engine. Fuel leakage can be surely prevented by pressing the spherical end of the metal pipe against a seat formed in the counterpart on the engine by the nut of the pipe fastening structure. Thus, the pipe fastening structure has been widely used in high-pressure fuel lines.
For example, pipe fastening structures disclosed in Patent documents 1, 2 and 3 are representative prior art pipe fastening structures.
Generally, each of the prior art pipe fastening structures includes a sleeve (or a washer) placed between a nut and a spherical end.
The sleeve is effective in applying high pressure to the spherical end to press the spherical end against the seat of a counterpart. When the sleeve is thus used, local pressure reduction can be avoided and stable sealing can be achieved even if the nut is fastened with the respective axes of the spherical end and the counterpart are slightly misaligned because the respective axes of the spherical end and the sleeve are aligned.
However, the number of the component parts of the pipe fastening structure increases, the nut becomes necessarily large and the weight of the pipe fastening structure increases necessarily when the sleeve is used.
Therefore, studies have been made to omit the sleeve and to fasten a metal pipe to a counterpart only with a nut and such pipe fastening structures have been practically used in recent years.    Patent document 1: JP-A H10-122454    Patent document 2: JP-A 2000-227183    Patent document 3: JP-A 2009-144668